Such a steering is, for example, known from DE 10 2004 021 531 A1.
Such a hydraulic steering is preferably used with tractors or other self-propelled working machines, which are driving in different environments. A tractor, for example, must drive in public streets to get to a field. Its real work is then done, when driving in the field. When driving in the street, a steering behaviour is desired that corresponds to that of an ordinary vehicle. This means that the driver must be able to feel feedback of the steered wheels on the steering handwheel. For this purpose, it is required that the steering unit has feedback behaviour. Such a steering unit then also has the advantage that the vehicle automatically returns to straight forward driving, that is, the steered wheels can, in a manner of speaking, automatically return to the straight forward position, which the driver learns by means of the turning of the steering handwheel. When driving in the field, it is in many cases desired not to have such feedback behaviour. Otherwise, the driver would constantly have to act upon the steering handwheel or another steering member with a certain force.